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Future Trends of Flexible Circuit Boards
Rigid-Flex PCB Stack-up for Impedance Controlled Designs
Control Impedance Between Rigid PCB and Flex PCB
Flex PCB Reliability and Bendability
Normal Flex PCB Specifications
Flex PCB Polyimide Coverlay and Solder Mask
Flex PCB Boards and Connectors
About RA Copper and ED Copper
Introduction of Flexible PCB
5 Tips For Designing Flexible PCB
Advantages of FPC (Flexible PCB)
Evolution of the Flex Printed Circuit Board
Benefits of Using Flex Circuit Boards
Why Rigid-Flex PCBs are Economical?
Flexible PCB vs Rigid PCB
Development of Flexible printed circuit board (FPC) market
Traditional Manufacture Engineering of FPC Substrate
Development Trend of FPC Board
Flex PCB and the Manufacturing
About Flex PCB design
About Flex PCB and Assembly
How to Ensure Flex PCB Design Success
How to Select the Appropriate FPC Materials?
The Differences In Rigid PCB, Flex PCB and Rigid-Flex PCB
Flex-Rigid PCB Design Guidelines
Beneficials for Polyimide Flex PCB Boards
About Stiffener on Flex PCB FPC circuit Boards
PCB Surface Finish Comparison
Copper Thickness for FPC Boards
Interconnect Solutions for Flexible Printed Circuits and Etched Foil Heaters
Advantages and Disadvantages of Rigid-Flex PCB
About FPC Plating Process
About EMI shield design for Flex Printed Circuit Board
PCB Assembly Blog
About PCB Assembly
QFP and BGA and the Development Trend in PCB assembly
Why some components need be baked before reflow soldering
About Flex PCB Assembly
Manual Soldering in SMT Assembly Manufacturing Process
BGA Components and BGA Assembly
Quick Understanding for PCB Assembly Process
About SMT Assembly (Surface Mount Technology)
About THT Assembly (Through-Hole Technology)
About Reflow Soldering
PCB Assembly Inspections and Tests
Panel Requirements for PCB Assembly
About SMT (Surface Mount Technology)



When it comes to flex circuits and rigid-flex PCBs, stiffeners are a common and important requirement in many designs as they improve the durability and reliability of the flex circuit in many applications. 


Stiffeners also provide a mechanical function for the flexible area and are not part of the electrical requirements of the overall part. If your board requires stiffeners on one or both sides of the flex, it may require multiple lamination cycles, which will add to the cost and the lead time. However, there are certain cases where stiffeners are ultimately required. 


Flexible PCB Stiffener Requirements 


Flexible PCB stiffener requirements fall under the following usage categories: 


1. Rigidizing connector areas for stress relief of larger connectors or repeated insertions of the connector.


2. ZIF (Zero Insertion Force) Thickness Requirements. 


3. Localized Bend Constraint(s).


4. Creating a flat surface for the placement of SMT pads and components.


5. Minimizing component stress Managing heat dissipation (aluminum and steel).


6. Make the array rigid enough to run through the automated assembly processes without a fixture 


Things to Know About Flex PCB Stiffeners


Stiffeners are added to the flexible circuit as one of the final fabrication steps and can be applied with either a layer of pressure sensitive adhesive (PSA) or a layer of thermal set adhesive. From a low-cost perspective, the pressure-sensitive adhesive application will most often be slightly less expensive. Thermal set on-flow adhesive (traditional PCB prepreg) requires the flex PCB to be placed back in the lamination press to apply the heat and pressure needed to cure the adhesive, which has to be pre-cut to the specific shape of the stiffener itself. 


Make sure when specifying PCB stiffeners to go along with plated-through hole components that the design will have the stiffener on the same side of the board where you’re inserting the component, so you can access the solder pads. In these applications, being able to accurately register the holes on the stiffener to the plated through holes on the PCB is critical to ensure that the component leads can be easily fed through the holes. As part of this process, customers need to work with their flex PCB fabricator in order to ensure that the adhesive system used, and the method of registration used will be adequate to accomplish the customer objectives. 


Adding stiffeners to flexible and rigid-flex PCBs will add extra processing time, material, and labor, which will add cost. As a design tip, stiffener and coverlay termination points on top and bottom layers should overlap a minimum of 0.030" to avoid stress points and keeping all the stiffeners the same thickness reduces processing time in fabrication. 


When choosing the thickness of the stiffener material for flex and rigid-flex PCBs the best method for reducing cost and lead time is to stick to the most common laminate thickness such as 0.010", 0.031", 0.047", or 0.062", just to name a few. Each fabricator has its own stock of different products, so it is important that you work with them to determine what the best common thickness is to ensure that you are not specifying a thickness that creates cost and logistics challenges. 


Why Use Stiffeners in Flex Circuit Application 


Flexible PCB stiffener requirements fall in the following usage categories: 


A1. Rigidizing Component / Connector areas 


B2. ZIF (Zero Insertion Force) Thickness Requirements 


C3. Localized Bend Constraint(s) 


A1. Rigidizing Component / Connector Stiffeners




1. Creates a localized rigid area where components / connectors are attached. 


2. Protects solder joints by preventing bending of the flex in component area. 


Material Options: 


. FR4, Polyimide, Aluminum, Stainless Steel 


. Variety of thicknesses available 


. Attachment methods: Thermally bonded with flex adhesive, or pressure Sensitive Adhesive (PSA) 



B2. ZIF Stiffeners/Golden Finger Stiffiners 



Localized thickness increase at contact fingers to meet specific ZIF connector specifications. 


Material Options: 


. Polyimide Only 


. Variety of thicknesses available to meet specific design requirements 


. Attachment methods: Thermally Bonded with flex adhesive only 


C3. Localized Bend Constraints 



Restrict bend area(s) to specific location(s) in a flex design to either facilitate final assembly, achieve specific bend requirement(s) or other end use requirement. 


Material Options: 

. FR4, Polyimide, Aluminum, Stainless Steel  


. Variety of thicknesses available 


. Attachment methods: Thermally Bonded with flex adhesive, or pressure Sensitive Adhesive (PSA)


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